Letters: 600 Kilo-what?

I don't understand the featured article on the Home Energy website this month (“Measured Performance of Heat Pump Water Heaters,” HE online Summer ’17). It claims that the average electric consumption of clothes dryers is about 600 kWh per year. My wife and I have a vented Kenmore natural-gas clothes dryer that I have separately metered. From December 15, 2013, through January 26, 2017, the meter registered 26 ccf (hundred cubic feet), or about 8.7 ccf per year. That would equate to about 260 kWh per year.